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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

feLACIERS OF PASSAGE CANAL AND BLACKSTONE BAY        557
with ablation moraine, which gives part of it a fictitious appearance of being lobate. A crescentic terminal moraine marks the site of a former extension of the glacier and inside of it is a barren zone, the largest in Blackstone Bay. It includes a large area of bare rock with scattered young shrubs, and outside of it is the normal mature forest.
There is a possibility that this glacier has had a double history of recent advances, for the part of the barren zone nearest the glacier had almost no vegetation and was separated by a small, low moraine from an outer portion which had a great many more young shrubs. The age of this youngest vegetation near Ripon and Lawrence Glaciers was not determined carefully, but may be estimated as certainly less than 20 years.
Glaciers on the Western Side of Blackstone Bay. The Northland, Milton, Concordia, and Carroll Glaciers are fed from n6v6 fields west of the Blackstone Glacier. None of them are as large as the glaciers on the eastern side of the bay, and all excepting the Northland Glacier terminate from a quarter to a half mile from the bay. Northland Glacier extends down nearly to the branch of the bay west of Willard Island.
All of these glaciers are without moraines, and are more or less crevassed, the Carroll, Concordia and Milton Glaciers, which are descending steeper slopes than the Northland Glacier, being most crevassed. There is not enough previous information about these glaciers to determine their recent history, although the sliding down of fragments from some of the termini, and the absence of barren zones, suggested the possibility of recent advance.
Glacial Erosion—Erosion Above Sea Level. Evidence of glacial erosion in Blackstone Bay is found in the form of the fiord above sea level, and especially in the steepened walls and bare rock slopes scored with glacial striae. Willard Island shows this glacia-tion particularly well, having numerous typical roches moutonn6e forms, and good-sized rock basins containing lakes. The northern wall of Blackstone Bay at the curve, eight miles north of Blackstone Glacier, is far steeper than the opposite wall, suggesting glacial undercutting on the outer side of this curve. There are also hanging valleys illustrated by the valleys of Carroll and adjacent glaciers, and by a valley on the eastern side of the fiord about three miles north of Ripon Glacier. Its lip is a few hundred feet about sea level, and it has no glacier at present.
Submarine Erosion. The portion of the fiord below sea level, as revealed by soundings, also emphasizes the great amount of glacial erosion. The fiord bottom slopes at the average rate of about 75 feet to the mile, having a depth of 216 feet near the Black-stone and Beloit Glaciers, and 1080 feet where Blackstone Bay enters Passage Canal 11^ miles distant. Half way down the fiord, however, there is a deeper point (1188 feet); and there is a steep descent of over 500 feet, 2$ miles from the head, besides other interruptions of slope, described below. The visibly steepened slopes of the side walls of the fiord seem to continue below sea level, suggesting that it has the normal relatively flat-bottomed cross section.
The slope of the bottom of t.hia fiord from Beloit Glacier to the southern end of Willard Island is at the rate of about 100 feet to the mile. Then comes a conspicuous increase in the slope with a descent of 462 to 522 feet in less than three-quarters of a mile, or at the rate of over 650 feet to the mile. Such a confluence step in the fiord bottom is a feature often observed in glaciated mountain valleys and the same condition exists above sea level at the southern end of Blackstone Bay where Blackstone and Beloit Glaciers descend a similar step of even greater height. A possible explanation of this